Spray painting has been performed in a process of painting automobiles, electrical apparatuses, metal products, and the like. Spray painting produces a large amount of overspray paint (excess paint), which is not deposited on the item that is to be painted. An amount of excess paint produced in such a painting process is about 50% to 60% of the amount of the paint used, excluding the case where electrostatic painting, which offers a high painting efficiency, is used. Accordingly, it is necessary to remove and collect excess paint from the environment of the painting process. Wet paint booths, in which water is used for cleaning, have been commonly used for collecting excess paint. The cleaning water is circulated and reused. In such a painting booth, excess paint contained in the circulation water is coagulated and separated in order to prevent the accumulation of paint that remains in the circulation water.
Paints are broadly divided into solvent-based paints that include only an organic solvent, such as a thinner, as a solvent and water-based paints that include water as a solvent. Solvent-based paints have higher weather resistance and higher chipping resistance than water-based paints and have been widely used particularly for automotive top clear coating. In the case where a solvent-based paint is used, the particles of excess paint mixed in the circulation water, which have high adhesion, are likely to adhere to and severely contaminate the facilities and may also coagulate with one another to form large flocs, which cause clogging.
Accordingly, it is important for coagulating water that circulates through a wet paint booth which contains a solvent-based paint to reduce the adhesion of the solvent-based paint.
It is publicly known that an aluminum inorganic coagulant can be used for treating water that circulates through a wet paint booth. PTL 1 proposes a technique in which a mixture of a copolymer of tannine and a cationic monomer with polyaluminum chloride is used for treating water that circulates through a wet paint booth which contains a water-based paint and a solvent-based paint. PTL 2 proposes a technique in which an inorganic coagulant is used in combination with a polymer coagulant aid for treating a water-based paint.
There have been cases where a clay mineral, a phenol resin, tannine, and the like are used as an agent for reducing the adhesion of solvent-based paints. However, the above agents need to be used in combination with a cationic chemical for performing the adhesion reduction treatment. PTL 3 proposes a method in which a phenol resin and a cationic polymer, such as an alkylamine-epichlorohydrin condensate, are added to water that circulates through a wet paint booth in a predetermined ratio.
While anionic chemicals, such as a phenol resin, have been known as an agent for reducing the adhesion of solvent-based paints as described in PTL 3, the use of anionic chemicals require a complex chemical control because they need to be used in combination with a cationic chemical and the amounts of the anionic chemical and the cationic chemical need to be controlled in order to keep the ionic balance in the circulation water.
PTL 1: U.S. Pat. No. 5,614,103
PTL 2: Japanese Patent Publication No. S52-71538
PTL 3: Japanese Patent No. 4069799